Reciprocating hermetic compressors are generally provided with suction acoustic dampening systems (acoustic filters), which are disposed inside the shell with the function to attenuate the noise generated during the suction of the refrigerant fluid. Such components, however, cause losses both in the refrigerating capacity and in the efficiency of the compressor, resulting from gas overheating and flow restriction. The manufacture of said filters from plastic materials have meant a significant advance regarding their optimization, although a considerable amount of the compressor losses is still due to this component.
In reciprocating compressors, the movement of the piston and the use of suction and discharge valves, which open only during a fraction of the total cycle, produce a pulsing gas flow both in the suction and in the discharge lines. Such flow is one of the causes of noise, which may be transmitted to the environment in two forms: by the excitement of the ressonance frequencies of the inner cavity of the compressor, or of other component of the mechanical assembly, or by the excitement of the ressonance frequencies of the piping of the refrigerant system, i.e., evaporator, condenser and connecting tubes of these components of the compressor refrigerating system. In the first case, the noise is transmitted to the shell, which irradiates it to the external environment.
In order to attenuate the noise generated by the pulsing flow, acoustic dampening systems (acoustic filters) have been used. These systems may be classified as dissipative and reactive systems. The dissipative dampening systems absorb sound energy, but create an undesirable pressure loss. On the other hand, the reactive mufflers tend to reflect part of the sound energy, thereby reducing pressure loss. The dissipative mufflers are more used in discharge dampening systems, where the pulsation is high. The reactive systems are preferred for the suction, since they present less pressure loss. Said pressure loss in the acoustic filters is one of the causes that reduce the efficiency of the compressors, mainly in the suction case, which is more sensible to the pressure loss effects.
Other cause that reduces the efficiency of the compressors, when usual acoustic mufflers are employed, is the overheating of the suctioned gas. During the time interval between the entrance of the gas to the compressor and its admission to the compressor cylinder, the gas temperature is increased, due to heat transfer from the several hot sources existing inside the compressor. The temperature increase causes an increase in the specific volume and consequently a reduction in the refrigerant mass flow. Since the refrigerating capacity of the compressor is directly proportional to the mass flow, reducing said flow results in efficiency loss.
Reducing these negative effects has been achieved with the evolution in the acoustic filter designs.
In prior constructions, the gas coming from the suction line and discharged into the shell passes through the main hot sources inside the compressor, before reaching the filter and being drawn towards the cylinder inside (indirect suction). This gas circulation should promote the cooling of the motor. Because of this and because the filters were usually metallic, the efficiency of the compressor was impaired due to gas overheating.
The requirements for more efficient compressors have led to the development of acoustic dampening systems with more efficient conceptions. The gas, rather than passing through all heated parts inside the compressor, is drawn directly to the inside of the suction filter (U.S. Pat. No. 1,591,239, U.S. Pat. No. 4,242,056). Other technique uses, in the suction piping inside the compressor, nozzles or flared tubes (U.S. Pat. No. 4,486,153), which allow the flow to be directed between the inlet tube and the suction filter. Moreover, such filters began to be manufactured with plastic materials, which have adequate thermal insulating properties. These improvements brought about considerable increases in the efficiency of the refrigerating hermetic compressors. Nevertheless, overheating and load loss due to the use of the suction filter still represent significant amounts in the efficiency losses of the compressors.
In the reciprocating hermetic compressors known in the art, the gas coming from the evaporator enters into the shell and then passes through the suction filter, wherefrom it is drawn to the inside of the cylinder defined in the cylinder block, where it is compressed up to a pressure sufficient to open the discharge valve. Upon being discharged, said gas passes through the discharge valve and discharge filter, leaving the compressor inside and leading towards the condenser of the refrigerating system. In this type of compressor, the discharge filter is always hermetic, i.e., the gas is not released into the shell inside, whereas the suction filter is in fluid communication with said shell inside.
The fact that the compressor has low pressure inside the shell brings about two negative consequences regarding its efficiency. During great part of the compression cycle, the gas inside the cylinder is at a higher pressure than that of the gas inside the shell. This pressure difference generates a gas leakage from the cylinder towards the shell inside, through the gap existing between the piston and the cylinder. This gas is then admitted again in the cylinder through the suction filter, in function of the pressure balance occurring between the shell inside and the cylinder. Such gas is at a higher temperature than that of the gas returning to the evaporator, which causes a reduction in the pumped mass explained above.
This reduction of the pumped mass causes loss of refrigerating capacity and of efficiency, as well (loss due to the leakage through the piston-cylinder gap).
The pressure difference between the cylinder inside and the shell inside also creates a force at the piston top, which is transmitted, through the connecting rod, to the eccentric and bearings. The intensity of this force determines the dimensioning of the piston and bearings: the higher said force, the larger will be the dimensions of said parts and, consequently, the larger will be the dissipation of energy or viscous energy loss in the bearings.